1. Field of the Invention
The present invention relates to apparatus for measuring the earth's magnetic field. More particularly, this invention pertains to apparatus for continuously orienting a magnetometer in such a way that the magnitude of a local magnetic field may be measured with maximum magnetometer sensitivity.
2. Description of the Prior Art
In recent years the optically pumped magnetometer has been widely used as a magnetic field sensing device, capable of highly accurate measurement (on the order of one part per million or better) of the earth's field. This instrument utilizes quantum mechanical effects to produce an output frequency responsive to the magnitude of the local magnetic field. While its principles of operation and details of construction are well known, it is to be noted that the instrument includes a sample vapor, usually cesium, through which a beam of light (known as a "pumping beam"), defining the optical axis of the magnetometer, is transmitted.
It is a characteristic of the single cell optically pumped magnetometer that the accuracy with which the magnitude of magnetic field is sensed is a function of the angular orientation of the instrument with respect to the local magnetic field. Commonly, optimum measurement accuracy is achieved when the optical axis of the device is oriented 45 degrees with respect to the local magnetic field vector. Thus the locus of optimum detection for such device consists of a cone of half angle of 45 degrees centered about the optical axis of the magnetometer. The alignment of the local magnetic field vector with any element of this cone satisfies the optimum detection relationship for the instrument.
In mapping the earth's magnetic field and in numerous other applications, the magnetometer is associated with a moving platform, such as an airplane, which will generally traverse a selected area in a preselected pattern. Thus, the magnetometer must be continuously reoriented throughout the mapping process to achieve an output of uniform sensitivity and accuracy. Numerous approaches to the orientation problem have been attempted. In one, six separate magnetometers are arranged along three axes that, in turn, are so arranged that the total field vector moves angularly with respect to the axes. In such manner, the measurement task moves successively from pair to pair along the axes, allowing the field vector to maintain a tolerable, though not optimum, angular relationship with at least one cell pair at all times. This produces an essentially omni-directional axis of sensitivity. Such configuration, while eliminating the need for a magnetometer orientation mechanism, is characterized by high costs of both manufacture and maintenance that result from the sixfold redundancy required of many of the system's components.
Various orientation systems utilize gimbal-type apparatus. Included among these are two systems marketed by Varian Associates. In one, a selected ray of the aforementioned cone of the magnetometer is maintained in an approximately optimal orientation with respect to the local magnetic field by manual rotation of the gimbals. The repointing of the magnetometer twists it about its axis. Such an orientation system is hindered in operation by relatively stiff cables normally associated with the magnetometer which require the application of substantial torquing forces to effect such twisting of the magnetometer. In a second system, a three axis gimbal is driven by a closed loop servomechanism that senses the direction of the earth's magnetic fields by analyzing the effects of an added, cyclically varied magnetic field. The servomechanism orients the appropriate gimbal to maintain alignment of the optical axis along the field direction. Although this apparatus provides high accuracy measurements, it is expensive, heavy and bulky, the cost of the orientation system being an order of magnitude greater than that of the magnetometer which it supports.